Excretory system of fish

• The excretory organs consist of kidneys, ureters, and urinary bladder.

a) Kidney:

• The kidneys of vertebrates are made up of nephrons or kidney tubules.
• In ancestral vertebrates, the kidney possesses one nephron for each of those body segments that lay between the anterior and posterior end of the coelom.
• The nephron drained into a duct called Wolffian or archinephric duct located posterior to the cloaca. This sort of kidney is known as holonephros because it extends to the entire length of the body.
• The kidneys occupy a dorsal position in the body cavity and are placed just ventral to the vertebral column. In teleost, the kidney is distinguished into head and trunk regions.
• The two kidneys are fused in the middle, forming a flattened wing-like middle portion of the trunk kidneys. From this wing-like structure, two tube-like structures are given out anteriorly which are separate from each other. At the anterior end of these tubes are present sac-like structures known as head kidneys.
• The two head kidneys are separate. Posteriorly, from the middle part of the trunk kidney, the kidneys become narrow. This part is called as tail kidneys. Externally, a depression is visible which indicates that they are two structures.
• The tail kidneys are also fused. The trunk kidneys are very big in comparison to the length of a fish. Dorsally, the trunk kidneys show several annulations whereas ventrally it is smooth in outline.

b) Ureter:

• The mesonephric ducts or ureters lie closed together in the median line. Anteriorly, they are separate, posteriorly the two mesonephric ducts open separately into the urinary bladder. In some species, a sac-like enlargement is visible in the posterior region of the ureter.
• This is known as the urinary bladder, but it is not homologous to that of higher vertebrates. The urinary bladder usually opens to the exterior by a common urinogenital aperture in’ male fish but a separate urinary aperture is present in female fish as found in Mystus.
• The function of the ureter is to conduct urine up to the urinary bladder.
• Histologically, it is made up externally by tunica adventitia, the middle layer contains lamina propria and smooth muscles and the outermost layer is columnar epithelial cells.